The present invention generally relates to apparatus and methods for providing protection for an electrical system and, more particularly, to apparatus and methods of ground fault detection in an ungrounded electrical system.
In a grounded three phase alternating current (AC) electric power system a neutral point may be connected to ground (or chassis ground). In a grounded direct current (DC) power system, either a positive or a return conductor may be grounded. In some cases, a middle point of a DC voltage source may be grounded.
A grounded system may be grounded solidly or through some impedance. Grounding through an impedance may help in controlling a level of fault current arising out of a single line-ground fault. An ungrounded system does not have any direct connection to the ground. In other words, the grounding impedance tends to infinity.
The purpose of grounding a generator or transformer neutral is to limit a voltage rise among healthy phases when single line to ground fault occurs. It may also provide a path for zero-sequence current which may help in detection of unwanted phase to ground connection in the system.
A disadvantage of a grounded system lies in the fact that even a single line-ground fault may lead to heavy fault currents and hence disrupt operation of the entire power system. The fault has to be cleared before the grounded system resumes its normal operation. Ground faults can lead to process disruption and safety hazards such as equipment malfunctions, fire and electric shock. During a ground fault condition, power supply has to be interrupted to limit the damage to equipment.
In an ungrounded electrical power system, there may be no intentional connection between the conductors and the ground. However, in any system, stray capacitive coupling may exist between the system conductors and adjacent grounded surfaces. Consequently, an “ungrounded system” is, as a practical matter, a “capacitive grounded system” by virtue of the distributed stray capacitance.
The advantage of ungrounded system is that a single line to ground fault may have minimal fault current and hence practically no impact on the system operation. However in the event of a line-ground fault in an ungrounded system, the voltage of the healthy phases may rise to line-to-line voltage. Thus voltage stress on the healthy phase conductors may increase during fault condition. Further, there may be capacitive voltage build up if the fault is of restriking nature. Thus the phase conductors have to be insulated by design for higher voltage stress in an ungrounded system. Also, due to increased voltage stress, the probability of occurrence of second line to ground fault may increase.
Though a single line to ground fault may not impact operation of an ungrounded system, a second ground fault may lead to phase-to-phase fault, with very high fault current. In the event of such a phase-to-phase fault, power interruption is required, thus leading to system outage. It is, therefore, desirable to detect and isolate and clear the first line-ground fault as soon as possible, even though it does not affect the system operation.
A single line-ground fault in an ungrounded system may produce very small current flowing through any shunt-connected or parasitic and stray capacitances. Therefore the fault detection and in particular localization is a daunting task in a large and complex ungrounded electrical distribution network which may include multiple power sources and utilization systems. Several methods have been proposed in the prior art for single line-to-ground fault detection in ungrounded or floating networks. Most of the ground fault detection methods described in the prior art rely on measurement of positive and negative line voltage with respect to a common chassis or ground potential. During single line-to-ground condition, the faulty terminal voltage may assume ground potential while the ‘healthy’ phase potential may rise to the original line-to-line voltage level. Single line-to-ground fault is declared when the difference between measured voltage at the positive and the negative line with respect to common chassis exceeds a threshold value. However, these methods fail to indicate the localization of the ground fault. Since it is not desirable to shutdown the entire network in the event of a single ground fault it would be a great advantage to localize the fault and isolate only the faulty section of the network while keeping the unaffected network functions operative.
Prior-art fault localization methods may include sequential disconnection of network sections until the fault is isolated. However, this procedure leads to unnecessary disconnection of power sources or loads, disrupting the system operation.
As can be seen, there is a need for a system that may provide for early detection and isolation of single line-ground fault in an ungrounded power distribution system. Furthermore, there is a need to such a system which may provide both detection and localization of a faulted network section.